The present invention relates to a heat-fixing device in which a toner image formed on paper or like sheets passes between heating and pressing rollers to be fixed to the sheet.
A conventional heat-fixing device includes a heating roller and a pressing roller. The heating roller includes a core which is made of materials with good thermal conductivity such as aluminum alloy, stainless steel or iron alloy, etc. and is heated by a heating source such as an infrared lamp, halogen lamp or Nichrome wire, etc. and an anti-adhesive and heat-resistant surface which is made of a fluorine contained polymer (PTFE or PFA or the like). The pressing roller is coated with an elastic heat-resistant material (silicone rubber or fluoro rubber, etc.). In the conventional heat-fixing device, if the pressing roller is always pressed against the heating roller, a contact portion therebetween, particularly a cross-sectional shape of the pressing roller deforms so that a smooth and effective fixing operation cannot be carried out. Therefore, the heating roller and the pressing roller contact with each other only when the toner image on the sheet such as plain paper, photosensitive paper, etc. is pressed therebetween, and are separated from each other when the sheet is not pressed therebetween or the sheet jams, as disclosed in Publication of Japanese Patent Laid-open No. 57-76582.
In the conventional heat-fixing device as shown in FIG. 2, the heating roller 103 with the heating source 102 contacts with the pressing roller 104 to form a nip portion 115 for passing the sheet (not shown) along the pass line 101 by rotation of the heating roller 103 and the pressing roller 104. An arm 105 is rotatable through a pin 108 fixed to an end thereof and supports a shaft 107 of the pressing roller 104 through a groove 106 at an intermediate portion of the arm 105. A rotatable roller 110 is arranged at another end of the arm 105 and is in constant contact with an eccentric cam 109. The pin 108 is movable in a guide groove 112 on a frame 111 and is urged by a tension spring 114 toward a hanger 113 so that the pressing roller 104 is pressed against the heating roller 103. When the sheet is pressed between the heating roller 103 and the pressing roller 104 to fix the toner image thereon, the largest radius portion of the eccentric cam 109 contacts with the roller 110 to press the pressing roller 104 against the heating roller 103. When the sheet is not pressed between the heating roller 103 and the pressing roller 104 or when the sheet jams in the heat-fixing device, the eccentric cam 109 rotates by 180 degrees from the position shown in FIG. 2 and the smallest radius portion of the eccentric cam 109 contacts with the roller 110 to separate the pressing roller 104 from the heating roller 103. Since the tension coil spring 114 extends between the pin 108 supporting the arm 105 and a hanger 113 mounted on a frame (not shown) to press the pressing roller 104, a length for the spring 114 is large, an excessive space is necessary, and a size of the device is large. And, because of a variation in length of the tension coil spring 114 and/or a jam in engagement between the pin 108 and the guide groove 112, there is a possibility that a pressing operation of the pressing roller 104 is not released when the eccentric cam 109 rotates to separate the pressing roller 104 from the heating roller 103.